It is known that carboxylic acids and carboxylic acid derivatives may be produced by the carbonylation of a mixture of (1) a carbonylation feedstock compound such as alcohols, esters, ethers or mixtures thereof and (2) a halide compound such as hydrogen iodide and/or an alkyl iodide. The products of these known processes are carboxylic acids, esters and/or anhydrides, depending on the specific feedstock and processes conditions employed. Carboxylic esters and anhydrides are the primary products when water is absent, whereas carboxylic acids and esters are the main products when water is present. The most efficient catalysts for these reactions are metals from Group VIII of the Periodic Table. Thus carbonylation of mixtures of methanol and a halide compound produces acetic acid, and methyl acetate and water also may be produced at low methanol conversion. Carbonylation of mixtures of methyl acetate and a halide compound produces acetic anhydride in the absence of water.
The use of dimethyl ether (DME—also referred to as methyl ether) as the feedstock compound in carbonylation processes produces methyl acetate and acetic anhydride at sufficiently high rates of conversion in the absence of water. DME is an attractive feedstock material for acetic anhydride synthesis because the recycle of acetic acid equivalents through the carbonylation reactor is greatly reduced. Although DME is an attractive feedstock material for the synthesis of acetic acid and derivatives in some ways, it has limitations due to its physical properties. For example, the boiling point of DME is −24° C. at atmospheric pressure. Therefore, DME either must be stored in special pressure vessels or in refrigerated vessels. Either of these options is expensive. An accidental release of dimethyl ether into the environment may result in a fire or an explosion. An alternative carbonylation feedstock compound is needed which avoids high recycle of acetyl equivalents and the troublesome physical properties of DME.